Aptitude - Problems on Trains - Discussion

Abisheks answer is realy nice. Thank you abi.

x-5 is nothing but here x refers to the speed of train and as both train and boy are moving in the same direction relative velocity comes into picture and here relative velocity is the difference of two velocities.

If train moving same direction = SUBTRACTION OCCUR.
If train moving opposite direction = ADDITION OCCUR.

Let the speed of the train is x km/hr
Then the relative speed is (x-5)km/hr
Therefore,
Time = 10sec
Distance = 125m
Speed = (x-5)km/hr
As we are calculating in m/sec so we have to convert km/hr to m/sec.
(x-5)*1000/3600 m/sec or (x-5)*5/18 {by cancelling by 200.
200*5=1000,200*18=3600}

Therefore ,

Speed = distance/ time
(x-5)5/18 = 125/10
x-5 = 125*18/10*5
x-5 = 2250/50
x-5 = 45
x = 45+5
x = 50
Therefore x is 50km/hr (speed of the train)

Hai friends I want to give small tips.

If there train passes in same direction just subtract it.

If train passes in different or opposite direction direction just add it. If you need to convert km/pr use this formula (a*5%18) for speed and m/sec the formula is (a*18%5).

Why you made it 18/5, earlier it was 5/18, please explain me.

Here x=45 is speed of per 1km. question is 5km/hr .
So
x-5=45
x=45+5
x=50
Thats all.

l=125m;
s=5*5/18;
t=10s;
l=t*s;
s=t/l;
s=125/10;
s=12.5*18/5;
the man was also going same direction so,
s=45+5;
s=50;

Let the speed of the train is 'x'
Length=125m
Speed=10s
--> as per "Length of the train = speed X time"
--> 125m = 10s.x
--> x=(125m/10s) X (360/360)=(360 X 125m)/3600s
--> =45000m/3600s = 45km/1hr

As the above value is relative with the speed of man whose speed of running is 5km/1hr also to be added for the exact speed of the train.

There fore..
-->exact speed of the train is = (45km/1hr)+(5km/1hr)=50km/1hr

I believe its simple and right answer.

Let relative velocity of train with respect to man be vtm,actual velocity of train Be vt and velocity of man be vm.
We know that vtm=vt-vm,
But from problem vtm is =length of train/time taken
=(125*18/5)/10
=45 km/hr.

From above actual speed of train is,vt=vtm+vm

=45+5=50 km/hr.